Salt bath hardening

Since a few of the bits in this bloody Hardinge apron project could do with hardening after they've been made and since I haven't yet built a decent toolpost grinder. I thought I'd try an experiment to see how much distortion I might get when hardening the steel that I'm using to make the gears and shafts from.
I'd read, via Tubal Cain and others, that salt baths can be very handy things to heat steel up in. They should be less likely to over/under heat bits of the part you are heating than an open torch and you shouldn't get the scaling problems you would in an open atmosphere.
The steel I'm using is EN24 aka AISI 4340. This steel is hardened by austenitizing at 802 Centigrade and quenching in oil and (in my case) tempering at 200 centigrade.
It just so happens that common table salt melts at 801 centigrade.
Armed with this knowledge and the fact that SWMBO had over-ordered on salt when making a batch of play-dough for some kids lead to today's experiment.
I measured the trial part as well as I could and noted the dimensions on the drawing I had made it from. Then set up a crucible on some fire bricks and surrounded it with the rockwool that I've used in the past for heating things in. A pound or so of salt was poured into the dried out crucible and set to heat. While that was warming up I set up a paint kettle of new 15W-40 motor oil on a gas ring for the purposes of quenching to the desired temperature.
I was rather worried that the oil was approaching its flash point and only heated it to 170 centigrade instead of the 200 that I had wanted... must get some heavier oil.
The pictures are here:-
http://www.test-net.com/hardening-test
I quenched the part, then hurried indoors to put it in the pre-heated kitchen oven for half an hour or so, then I let it cool. Once I'd cleaned off the sticky oil blacking and the remaining specks of salt, I re-measured it. It looks like the biggest change in dimension is of the order of about a tenth of a thou per inch. I can live with this with no problems at all.
Having a pot fill of red hot liquid with nothing to stop it from tipping over is a little bit scary. I think that a properly built salt bath is definitely there in the future. I was very impressed by the complete lack of scale on the steel.
I don't yet know whether I achieved the hardness that I was aiming for. The part isn't file-hard, but is very hard. This is what I was expecting, since I was aiming at about 42HRC. I'll see if I can get it tested at work tomorrow to put my mind at rest.
It warmed the shed up as well :-)
I must put a heat spreader under the firebricks before I do the next one though. I ended up with the poor old workmate smouldering before I'd finished.
Mark Rand RTFM

Mark, At the risk of going off at a tangent and risking ridicule, you want a tool post grinder? I got mine by accident. I bought one of these Bosch POF 50 routers to do woodwork. The blurb, however, said that it had been developed from ---a tool post grinder!
OK, in the intervening years, more and more routers have been made and - no doubt, Robert Bosch has changed models, but with either a 1/4" or 6mm chuck and a 43mm collar most can be used similarly. Before finally castigating me because the bearings might need a possible change, you have one cheap machine.
Going off at a further tangent, Derek Brook's used a bigger one as his motor in his excellent tool and cutter grinder. Yes, I have the drawings- ex MEW.
Maybe, they are listed for £3.75. Yak, yak!!!!!!!
Norm

It sounds exciting - verging on dangerous ! I have a couple of thoughts.
I don't think you need to do the quench and temper in one go, temper at leisure. No need to run to the oven.
I have seen salt baths used in HSS heat treatment, I think the steel was quenched in the salt to about 550C and then held there. I don't remember salt being used for austenitising, and given the lack of scale formation, you would think its attractive, so there may be a hidden downside.
You don't say how long you left it in the salt bath, but it will need a while for the heat to get in (depending on the size of the item). Finally, distortion is often related to the orientation when you place it in the quench - so always quench a shaft end-on for example, not on its side.
But do take care, you have all the ingredients of a nasty accident. Fire, hot oil, hot metal, and molten salt - not to mention using the domestic oven to temper something you have just quenched in an oil bath !
Steve

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Tempering at leisure from a room temperature quench can be risky if there are any sudden changes of section in the part or if there is any chance that the part may be dropped. Tempering immediately is recommended in all cases.
Ideally I should have quenched to 297Centigrade for this particular alloy and used a salt bath for quenching since oil is a bit tricky at that temperature.
It would appear that the use of salt baths for both austenitising and high-temperature quenching/tempering is not uncommon out there in industry these days. The use with HSS is significant due to the requirements for accurate temperature control and multiple step heat treatments. The hidden downside is the problem of not handling or spilling the watery fluid that is hot enough to melt aluminium in.
The heating process is quite gentle to start with as the part gets covered in a layer of solidified salt when introduced into the bath. This slows down the heat transfer and helps to even-out the heating. once this layer has re-melted, the heat capacity and viscosity of the salt are similar to those of water, so the heating can be quite rapid. This particular part had a larges section of 3/16", so could be assumed to heat through quite rapidly.
On the safety side, There are significant dangers inherent in the setup I used and great care was taken to ensure that nothing was knocked over. Palling of the concrete floor would have really pissed me off, not to mention what first degree burns would have done to my mood. The shed has appropriate, good sized, fire extinguishers (dry powder, foam and CO2) to hand for the various eventualities.
I think that the future holds a couple of permanent, solidly built and thermostatically controlled salt baths. one chloride/carbonate for heating and one nitrite/nitrate for quenching. All in all I think this method is much more civilized than the old firebricks, torch and ruin the surface method :-)
Testing the part at work showed HRC54, which is right where I wanted it. :-)
Mark Rand RTFM

Mark Rand wrote:
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The critical thing about quenching steel is the cooling rate, not the end temperature. The target is the right transformation product, the cooling rate needed is governed by the steel grade.
The critical thing about tempering steel is the temperature (and time to a lesser extent) - not the cooling or heating rate. If you got a the right transformation product then you can use tempering to improve the toughness at the expense of hardness.
You seem to be confusing the two when you say you should have quenched in a 297C bath, as you are trying to determine your cooling rate by the quenchant temperature alone, whereas the cooling rate is determined by the section of the piece as well as the type and temperature of the quenchant. I am not sure if you have picked up an idea from a specific industrial heat treatment, but trying to quench EN24 in a 300C bath could be very difficult to predict what you will get and very sensitive to the cross section.
Plus - if someone is telling you you need 297C (as opposed to 300C) in any sort of steel heat treatment then they are taking the piss.
If you are worried about thermal stresses cracking complex components you are making, then are you sure you should be using EN24, maybe you would be better with case hardening of a milder steel.?
Steve

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Both are significant if you want a specific outcome.
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No one said it wasn't.
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Not in the least. To get the outcome that I would have liked, I should have quenched to 297C at a sufficiently rapid rate, end of story. I was not trying to modify the cooling rate by temperature. I _should_ have used a salt quench, both for a better cooling rate and to get the temperature I was aiming at, but got acceptable results with a less than satisfactory setup.
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Ms is 297 for that particular alloy. I don't see the significance of 300C. They are both quoted to three digits and I missed both by a long way, but there would be no benefit in aiming at the wrong value!
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I'm not worrying about thermal stresses cracking complex components. I'm not using techniques that would encourage that. :-)
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Mark Rand RTFM

Apropos the previous discussion, you are going down a very tricky route. In trying to quench to or above the Ms (the temperature that marks the start of martensitic transformation) you are attempting austempering. I doubt EN24 is suitable for this, and for the few alloy steels where it can be done it requires quite precise control of time and temperature.
To achieve austempering you need to cool the steel fast enough down to about 400C to avoid the pearlite transformation. As discussed this depends on the steel section, the steel grade as well as the stuff you quench it in. Then you are trying to arrest the cooling in the 300 to 350C range and wait for the austenite to transform to lower bainite or similar. You need a steel that will transform in a reasonable time - I am not sure if you have a TTT curve for the EN24, but you'll find you have a log axis for time. You may need to get it below 400C in a few seconds, and then wait hours at 300C. Its damned tricky to know if you have waited long enough to get this lower temperature transformation - and again the steel grade is critical - in fact everything is critical.
Given this is a very ambitious thing to do in industry, then its more than a little bit tricky to do with an amateur setup. But have fun trying anyway - just don't have a nasty accident.
Normal heat treatment for a carbon steel is to quench right through the Ms - get the rock hard martensite, then temper it back until you get the right combination of hardness and ductility (impact resistance). If your salt bath gives a good way of doing this without scale then thats interesting - and quite complicated enough.
Steve

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Yes to all of that.
The most significant thing for me was to minimize distortion. I'm quite happy with the results I obtained this time. For the rest of the job I'll probably go back to a room temperature oil quench and a 200C temper. I think that the best thing that has come out of it is the knowledge that I can now heat the parts cleanly without building a new muffle every time or using foil packaging.
For the longer term, I've got a 2' length of 8" bore 3/8" wall pipe that I can be cut into two and weld bases into the halves. Then with a bit of fabrication work, a bit of electronics, a lot of insulation and a lot of gas or electric, I can have can have a couple of baths. They will make predictable, safe hardening as simple as remembering to wheel the baths out from under the bench and turning them on an hour beforehand.
Mark Rand RTFM

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My first post on this group; been lurking for a few months.
Salt baths are, and have always been, used frequently in the aircraft industry. The reason being, the power/weight ratio thing demands the highest performance from high tech metals and their alloys. Frequently, to work the material, for fabrication, or repair, the state must be changed to one condition and then another for "in use."
The process was considered so "scary" that a separate shop was set up with no access from "the bay." Jobs had to be "handed in." And that was my recollection from the 1960's when real men used Carb Tet to shower with and Trichlorethelene to brush their teeth; youth of today don't believe a word we say :) Elf&safety, bah.
Not to overstate the case, but the old boys used to say it was safer with a gash bucket of molten lead for casting fishing weights than to be around molten salt baths.
I know that I would build something really solid and stable. Also, I would try to make sure that I was not to be "interrupted" during the whole process.
John

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It is, if the bucket doesn't collapse under the weight. Not least because the melting point of lead is only 327C compared with the 825C that I was aiming at. Also, if they were using cyanide baths for nitriding there are another set of things to worry about apart from being set on fire!
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Hah! It's hard enough to get her to bring me a cup of tea when I need it, let alone coming in the shed when there are evil glows, evil roaring noises and evil smells emanating from it. Obviously it's worse still once I actually start heating things and the effects are not just due to me :-)
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PS. Welcome to the club. Come in, spit on the cat and call the rug a bastard. They are a good lot here.
Mark Rand RTFM

JC Morrice wrote:
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I have dealt with molten lead. It behaves in a predictable way, and, provided you do not overheat it so that it boils, and you keep your fingers out of your mouth, it poses few risks. (Don't lick it, sniff the fumes, or stick your fingers in it. Pretty much common sense)
On the other hand, I have heard some stories (and seen the odd burn scar) from gunsmiths that used molten salt baths for blueing firearms parts. Between the funky chemistry (heavy metal salts in particular), and having to add water to the salts to keep the temperature in the correct range, with the attendant risk of it burping out blobs of molten salt when the water is enveloped in the salt and boils rapidly, I can understand the logic behind the guys thinking the salt bath was the greater risk.
Cheers Trevor Jones

Gents
I have read this thread with great interest. May I ask some questions?
Table salt melts at 801C so should be ok to harden EN8 without loads of scaling? What sort of crucible do you use and do you have to empty the molten salt out before it cools to down solidify? Do these crucibles stand this treatment ok, ie last for more than one or two attempts? I have been checking the domestic gas oven which looks ok for tempering. :-)
Steve
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EN8 is 830C to 860C so table salt should do the job easily.
The crucible I used was a straight clay crucible because I really didn't want to mess up my large Morganite Silicon Carbide crucible. There is quite a lot of shrinkage on solidification of the salt and I don't yet know whether the crucible will survive a re-heat. That will only be done outdoors on a nice summery day next year (if there is one) over a sand trap. I've got another 35 to go before I have to reuse one:-
http://www.test-net.com/workshop/inside_shed_small/pit1.jpg
If I build a permanent bath with a 3/8" wall steel pot, then I won't worry about splitting the thing on a re-heat. (see my latest reply to other Steve)
Tubal Cain goes into a reasonable amount of detail in his "Hardening, Tempering & Heat treating" book and even describes ways of constructing electric muffles and salt baths.
I've _always_ used the domestic electric oven for tempering and haven't had too much grief from the management. My memories of when we had a gas oven are that its temperature control was superior to most electric ovens, even if you needed a thermometer to see what temperature you had got. Given that we are mostly hobby folk, I can see a lot of benefit in chucking the odd tap or die in with the roast parsnips on a Sunday :-)
Have we got too many steves? <G>
Mark Rand RTFM

Mark Rand wrote:
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Yes we have :-)., but then there's safety in numbers. Wait until we get into a thread where we are arguing with ourselves, that will be really confusing !
I suppose we'll all have to think of distinctive signatures to pick us out - tricky !
Steve << Insert distinctive one liner here >>

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Ok! How many of us are there?
Steve R. On the storm battered West Coast of Vancouver Island, Canada.

Steve R. wrote:
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Too many. I was going to sign off Steve R, but saw it was taken !
The other Steve R. In the foggy NW of England

And me who was asking the questions: - Steve A from frosty, but sunny (this morning anyway) Sheffield, South Yorkshire, England.
Thanks to Mark Rand for his reply. It would seem to be wise to use a steel pot, as this type of salt bath is not recommended by the medical profession for soaking your sore bits in! Thanks also for the workshop pictures, I recognise the 'undercroft'
Will look at Tubal Cain's book
Steve A
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Hello all
just to go off at a slight tangent from the present topic.
Has anyone here got any thoughts on temperatures etc for hardening an tempering gauge plate to make new comb teeth for musical bo restoration work.
I have just got a 1200 degrees Celcius muffle furnace to do the job and wondered if any one else had done some experiments before I waste lot of time doing test pieces. Thanks Phi
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